Parallax correction in color television



Sept. 18, 1951 s. HANSEN 2,568,448

PARALLAX CORRECTION 1N COLOR TELEVISION Filed sept. 2s. 1947 Hi AAttorney `Patented Sept. 18, 1951 UNITED STATES PATENT OFFICEv PARALLAXCORRECTION IN COLOR TELEVISION Siegfried Hansen, Los Angeles, Calif.,assignor to General Electric Company, a corporation of New York 4Claims. l

This invention relates to improvements in television tubes of the typecomprising a plurality of alternate primary color strips on either thescreen of a receiving tube or the color filtering screen of atransmitting tube and a plurality of grid wires arranged to direct anelectron beam alternately and selectively to the color strips. It hasfor its general object the provision of a screen construction whichavoids parallax errors in tubes of that type.

The invention itself together with further objects and advantagesthereof will best be understood by reference to the followingspecification when taken in connection with accompanying drawings inwhich the Fig. l represents schematically a prior art tube constructiono i the aforementioned type which illustrates the parallax errors whichit is the object of the present invention to correct; Fig. 2 representsschematically television tubes have a grid and color strip arrangementsimproved in accordance with the invention to avoid parallax errors,while the Figs. 3 through 8 illustrate various steps in one process ormethod for constructing the improved color strip arrangement of the Fig.2.

The Fig, 1 represents in schematic cross section a typical receivingtube of the type now known in the art (cf. British Pat. No. 443,896-

General Electric Co., Ltd. and Leslie Connock Jesty) and may comprisegenerally, an electron gun I, a conventional vertical-horizontaldeflection plate system IA, a grid 2 and a picture screen 3 comprised ofa plurality of threads or strips 4 of fluorescent material of` differentcolors arranged in parallel configuration (i. e. normal to the plane ofthe ngure) on a glass plate 5. These fluorescent strips may comprise anysuitable fluorescent material capable of fiuorescing under impact ofelectrons in suitable colors (preferably'the primary colors green, redand blue) and distributed in the order indicated by the initials G, Rand B in the Fig. l. The grid 2 may comprise a series of parallel wiresarranged in a configuration generally parallel to the screen 3, each ofthe individual wires being parallel to the colored strips of the screen.As will be noted from the gure, all of the green strips of the screenare aligned with the gaps between the individual grid wires; all of thered strips are aligned with the alternate grid wires G which areelectrically common by virtue of the common lead 1; while all of theblue strips are aligned with the intermediate grid wires 8 which aresimilarly electrically common by virtue of their connection to the lead8.

In the operation of devices of the type shown in the Fig. 1, it hasheretofore been assumed that an electron beam which is adapted to excitethe picture screen 3 approaches that screen normally, i. e. as thoughthe distance from gun I to screen 3 were great enough to cause the beamto strike the screen at a substantially angle at all portions of thescreens surface. In that event, if there be no potential diierencebetween the grid wires E and the grid wires 8, all electrons of thestream will pass in substantially straight lines centrally through theinterstices between the grid wires and impinge only upon the greenstrips I. If the alternate grid wires 6 have a positive potentialimposed upon them with respect to the intermediate wires 8, then theelectron beam will be focused to impinge only upon the red strips.Similarly, if that potential relation be reversed and the wires 8 mademore positive than the wires 6, then the electron beam will be caused tofocus on the blue strips. In a manner readily understood by thoseskilled in the art, the entire screen may be scanned by an electron beamby appropriately varying the potential difference between wires 6 andwires 8, the beam may be caused to scan in repeating cycles first thered series of strips, then the green series and then the blue. Thelatter effect may be accomplished by imposing a suitable potential waveupon the wires 6 and 8 by a suitable voltage source Ill which may be arelaxation oscillator or similar device producing a block or squareshaped wave of the type shown graphically within the block.

It has been found that the assumption of normal approach of al1 electronparticles of beam to the color screen is not suiliciently accurate inpractice and will give rise to undesirably large parallax errors whenthe beam is emanating from point source such as point Il representingthe center of deflection of the conventional beam deflection systememployed and positioned at the usual finite distance from thegrid-screen structure. The error thus introduced may in practice besufficient to cause considerable distortion in the proper colorrelationships of the picture and may be viewed as being caused in thefollowing manner. Assuming that there is no potential difference betweenthe grid wires 6 and 8 and thattherefore the beam is intended to strikeonly the green strip, it will be noted that for the outer peripheralportions of the screen 3, the beam in traversing the interstices betweenany two grid members may be directed at the blue or red strips ratherthan the green because of the angular relations involved, This isillustrated for example by the dotted line I2 repre .3 senting oneillustrative position of the beam. It will be noted that because of theangular relationL caused by the proximity of the point II to the screenstructure, the beam has been directed.

, of deflection will usually be the same regardless of the direction ofthe beam path across the screen.

It will be understood that the arrangements of the Figures 1 and 2 willbe enclosed in any of the suitable envelopes known for cathode-raydevices y of this nature in the art. The screen 3 may comployed inpractice this parallax effect may be y enough to introduce substantialdistortion.

In order to avoid the mentioned parallax effects, there is provided thearrangement shown in Fig. 2 wherein the color strips of screen 3 arepositioned in more widely spaced relation than the Wires of grid 2 inorder,to.compensate for the divergence of the beam especially in theseouter portions. It will be noted that in this case the red strips 4 arealigned with the wires 6 of the grid but not by way of parallel lines asbefore. In this case, they are all aligned along radii (illustrated bydotted line I3) emanating from the point source of the electrons at thepoint II and each passing through one of the grid wires 6 and thecorresponding red strips 4. Substantially, the same arrangement isemployed for the green strips 4 which are similarly aligned with theinterstices between the grid wires along radii (illustrated by dottedline I4) extending from the point II and through those interstices. Thesame arrangement is made with the blue strips 4 which in this case arealigned along radii (illustrated by dotted line I5) through the gridwires 8 and converging at the point II. The effect is, therefore, toincrease the pitch of the screen (the spacing between the centers ofadjacent color strips) with respect to the pitch of .the grid wires(spacing between centers of adjacent wires) by a factor determined bythe relative spacings of point Il, grid 2 and screen 3. The relationbetween the pitch of the grid wires and that of the screen color stripsmay be represented in general form by the following formula representingthe situation -for a parallel grid plane configuration andv a parallelcolor screen:

Where: l

Ps=the pitch of the screen color strips Pg=the pitch of the grid wiresA=spacing between grid and screen B=spacing between center of deectionand screen the case of the conventional electrostatic deilecl tion platesystems, the center of deflection of that set of plates which sweeps thebeam in the direction normal to the length of the color strips and gridwires is the one of importance to the invention. For magnetic deflectionsystems, the center prise either lthe end face of such a tuoe or it maycomprise a separate glass plate mounted therewithin. The grid 2 may bemounted in any suitable manner within the tube in parallel relation f tothe screen.

'Ihe following described process will be found to be one particularlysuitable to the construction of parallax corrected color strip screensof the type of Fig. 2. A principal advantage lies in the fact that itobviates difficulties inherent in conventional methods of measuring andcontrolling the very small differences in pitches required by expression(1). As a rst step, there may be manufactured a set of printing platesfor printing in three steps each of the three series of color stripsupon plate 5. The printing'plates may be fabricated by a method similarto that used in photo-engraving, and which may be illustrated by theFigs. 3 and 4. For the first printing plate which is to be used forprinting the green strips 4 of Fig. 2, a glass plate I9 provided with aseries of fine parallel lines 20 having the same relative position andspacing as the interstices of the wires of grid 2 (e.g. 0.005 lines with0.005 spacing) is positioned between a point source of light 2I and aphotographic plate 22 which may be of glass or similar material providedwith a coatingV 23 of any suitable photosensitive emulsion. By pointsource of light is of course, meant any source having as littledimension as practicable except along the direction parallel to thecolor strips or grid wires, i. e. a point or line source. The linedglass plate I9 will be positioned with respect to thelight source 2I ina position corresponding to that which the grid structure 2 would havewith respect to the point II in the Fig. 2 with lines 20 correspondingin position to the grid wire interstices. Similarly, the photographicplate,22 will be in a position corresponding to that of the screen inFig. 2. The spacings are not necessarily identical with those in thefinal tube structure, but must be in the same relative proportion aftertaking into account all contributing factors such as index of refractionin the glass, etc. It will be apparent, therefore, that if the lightfrom the light source 2I be allowed to illuminate the photographicplate, there will be formed thereon a negative having translucent lines(unexposed portions) corresponding to the desired distribution of thegreen color lines on the screen 3 of Fig. 2. The positions of the latterwill be precisely those desired for the correction of the parallaxerror. After exposure and development the glass photographic plate 22with its negative may, as indicated in Fig. 4, be imposed upon a copperprinting plate 24 which is provided with a photo-sensitive emulsion lm25 which renders plate 24 acid resistant at any points which have beenexposed to light and yet capable of being acid etched at unexposedpoints. Plate 24 may thus be of the type common in the photo-engravingart and the etching process there employed may be used. If a print ofthe negative on plate 22 be made upon the film 25 by exposure to lightthere will be formed on the lm 25 print of a series of lines (exposedportions) corresponding in position to the desired position of the greencolor strips 4 in the screen of Fig. 2. By appropriate acid treatmentthe intermediate unexposed portions may be etched away in the usualmanner and there will remain at the exposed portions of the copper plate24 a series of elevations or ridges 2B in the coniiguration of parallellines corresponding to the desired position of the green color strips 4.The copper plate in this form is indicated in the Fig. 5. the ridges 26being positioned in the green strip configuration and interveninggrooves being in positions corresponding to the positions of the red andblue color strips 4 of screen Blof Fig. 2.

The copper plate 24 may be employed to print the color strips of greenfluorescent material upon the plate 5 of the Figs. 2 and 6 in thefollowing manner. The ridges 26 may be thinly coated with a suitableadhesive material to which a phosphor powder will adhere and the plate24 lmay be pressed against the glass plate 5 thereby leaving thereon aseries of lines of the adhesive material in proper positions. Thereafterthe plate 5 may be dusted with the appropriate phosphor material whichwill adhere to the adhesive material as a binder. Excess phosphormaterial may then be removed by use of a iine brush or by blowing withan air stream.

The width of the ridges 26 may be readily controlled during the etchingprocess in order to insure that they are neither too narrow nor too wideto form a color strip print of the desired width. If the acid etchingaction be of short time duration, the grooves between ridges 26 will besmall and consequently the ridges will be wide. Conversely, if theaction be of long time duration, the grooves will be large and theridges of correspondingly narrow width. The manner of control of etchingtime to this end is well known in the art.

The same method may be employed for printing the red and blue lines uponthe plate 5. For those cases, there may be prepared one or more printingplates similar to plate 24 but having a series of elevations or ridgescorresponding in position to the position of the red and blue strips ofthe screen 3 in Fig. 2. A separate plate may be prepared for each of thered series and the blue series of lines if desired but as will beindicated below a single reversible plate may be made to serve thepurpose of both. Such a plate is shown in the Figs. 6 and '1 as plate 21having ridges 28. In the Fig. 6 it is shown as being employed to printthe blue lines; while in the Fig. 'Z it is shown as being employed in aposition inverted with respect to that of Fig. '1 to print the redlines. As will be observed from a study of the color strip coniigurationof the preceding figures, a plate of this nature must be capable ofprinting a blue line in each of one series of alternate gaps between theuniformly spaced green lines; while for the red lines the plates must becapable of printing a red line in each of the remaining series ofalternate gaps not occupied by the blue lines.

Thus, the plate 21 will obviously have a spacing between ridges 28 whichis exactly twice the spacing of the ridges 26 on plate 24 of the Fig. 5employed to print the green lines. The Fig. 6 indicates the plate as inthe proper position to print the blue lines in vone series of alternatespaces between the green lines in the pattern corresponding to that ofFig. 2. In the Fig. 7, the plate has simply been inverted with respectto its Fig. 6 position whereby the order of the ridges 28 of the plate21 is transposed to correspond to the other series of alternate spacesbetween the green lines thus corresponding in position to the positionof the red lines in the Fig. 2.

The manner oi' preparing the plate 21 employed in the Figs. 6 and '1 issubstantially the same as that employed to prepare the plate 24 for theprinting of the green lines and the iirst step is indicatedschematically in the Fig.' 8 which corresponds to that of Fig. 3. Inthis case, however, a lined plate 29 corresponds to the plate I9 of theFig. 3 but has a series of ne parallel lines 30 with three times thespacing between them as was the case with the plate I9 (e. g. 0.005 inchlines with 0.015 inch spacing). The lined glass plate 29 may bepositioned with respect to the light sourcp 2l in a positioncorresponding to that which the grid structure 2 would have with respectto the point i l in Fig. 2 with the lines 30 corresponding in positionto the grid wires 8. Similarly the photographic plate 3l having emulsioniilm 32 will be in a position corresponding to that of the screen inFig. 2. It will be apparent therefore that if light from the source 2lnow be allowed to illuminate the photographic plate, there will beformed thereon a negative having translucent lines (unexposed portions)corresponding to the desired distribution of the blue lines of thecolored screen 3 of Fig. 2. The position of the negative lines will beprecisely those desired for the correction of parallax error. From`already described in connection with the printing of the green lines. y

The red lines may be printed upon the plate 5 in substantially the samemanner. If desired a separate printing plate 21 could be prepared forthe red lines simply by inverting the lined plate 29 of Fig. 8 andpreparing a negative which would then have lines corresponding to thered color strips. However, it will be observed that the preparation of aseparate plate for the red lines is rendered unnecessary by virtue ofthe fact that the plate 21 used to print the blue lines may simply beinverted and used as the printing plate for the red lines because theinversion will bring the ridges 28 into the proper order orconfiguration corresponding to the proper position of the red lines.

The method of imprinting color strips on,

out departing from the invention in its broader aspects, and therefore,the appended claims are intended to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electrical beam deflection device for color television comprisinga color screen having a plurality of intermeshed sets of different colorstrips. the strips within each set being of the same color, a source ofan electrical particle beam adapted to be deflected about a center ofdeilection to scan said screen and a control member positioned betweensaid screen and said center of deflection including a plurality ofelongated control members positioned between said screen and said centerof deflection fordetermining the set of color strips on which the beamimpinges when scanning the screen, the elongated conductorscorresponding in number to one set of strips of said screen andsupported in spaced apart relation, the space'between adjacentconductors bearing the same ratio to the spacing between adjacent colorstrips of one set as the distance between the center of defiection ofthe beam and the control member bears to the distance between the centerof deilection of the beam and the surface of the screen whereby parallaxerrors in the impingement of the beam on a selected set of color stripsis substantially eliminated.

2. An electrical beam deilection device for color television comprisinga color screen having three intermeshed sets of different color strips,the

strips within each set being of the same color,

a source of an electriealparticle beam adapted to be deflected about -acenter of deflection to scan said screen, a control member positionedbetween said screen and said center of deflection comprising two sets ofelongated conductors with each set insulated from the other, theconductors of one set being supported on radii extending from the centerof deflection to one set of color strips, the conductors of the otherset lying on radii extending from the center of deflection to a secondset of color strips and the third set of color strips lying on radiipassing from the center of deflection and through the spaces between thetwo sets of conductors.

3. An electrical beam deflection device for color television comprisinga color screen having a plurality of inter-meshed sets of differentcolor elements, the elements within each set being of the same color, asource of an electrical particle beam adapted to be deflected about acenter of deilection to scan said screen and a control member positionedbetween said screen and said center of deflection including a pluralityof electrostatic control portions positioned between said screen andsaid center of deflection for determining the set of color elements onwhich the beam impinges when scanning the screen,

the electrostatic control portions corresponding in number to one set ofelements of said screen and supported in spaced apart relation, thespace between adjacent control portions bearing the vsame ratio to thespacing between adjacent color elements of one set as the distancebetween the center of deflection of the beam and the control memberbears to the distance between the center of deflection of the beam andthe surface of the screen whereby parallax errors in the impingement ofthe beam on a selected set of color elements is substantiallyeliminated.

4. An electrical beam deflection device for color television comprisinga color screen having three inter-meshed sets of different colorelements, the elements within each set being of the same color, a sourceof an electrical particle beam adapted to be deflected about a center ofdeflection to scan said screen, a control member positioned between saidscreen and said center of deilection comprising two sets of conductorswith the conductors of each set connected together and insulated fromthe other, the conductors of one set being supported on radii extendingfrom the center of deflection to one set of color elements, theconductors of the other set lying on radii extending from the center ofdeilection to a second set of color elements and the third set of colorelements lying on radii passing from the center of deilection and thespaces between the two sets of conductors.

SIEGFRIED HANSEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Great Britain Nov. 5, 1935

